Ignition of thermit mixtures



Aug. 19, 1941. w, COHEN 2,253,364

IGNITION 0F THERMIT MIXTURES Filed July 2'7, 1940 fiwnr; E M 65/76 2 4W0 niting Thermits.

ders are still used, although some improvements Patented Aug. 19, 1941.UNlTE D STATES PATENT OFFICE romron or THERMIT vnx'rrmns Frederick W.Cohen, Upper Montclair, N. 8., as-

signor to Metal & Thermit Corporation, New York, N. Y., a corporationorNew Jersey Application July 27, 1940, Serial No. 347,965

'4 Claims.

' This invention relates to ignition of Thermit mixtures; and itcomprises a novel method of igniting Thermit mixtures wherein amass ofThermit powder is placed in a crucible and ignited across substantiallythe entire top of the mass, this being usually accomplished with the aidof an easily-combustiblaignition powder, whereby a self-propagatinghorizontal zone of ignition is formed having an area substantiallycorresponding to the cross section of the crucible, said zone travelingdownwardly to' the bottom of the crucible at a substantially uniform andcontrolled rate, thereby producing a highly eflicient reaction withreaction products at a maximum,

' uniform temperature; all asmore fully hereinafter set forth and asclaimed The word Thermit" is a trade-mark, owned by Metal & ThermitCorporation of'New York, used on mechanical mixtures of metaloxides andmetals, such as iron oxide and aluminum, the metals employed having ahigher amnity for oxypresent reduces or de-oxidizes the metal oxideproducing free metal, usually in the molten state, aswell as a metaloxide slag derived from the metal originally present.

Probably the most important. early contribution to the alumino-thermicart was that which is described in the early U. S. patent to HansGoldschmidt, No. 578,868. In this patent the patentee acknowledges thatthe alumino-thermic reaction was well known and had been used previouslyfor producing metals and alloys. He mentions the fact that the reaction,as then conducted, was 'very violent causing the loss of material fromthe crucible and waste of fuel used Since Hans Goldschmidt there hasbeen little r if any improvement made in the method of ig- TheGoldschmidt ignition powhave been made inthese powders, and the coldThermit mass is still ignited at one point, usually gen than the metalof said metal oxide, whereby upon ignition of the mixture the metaloriginally.

in the top of the crucible. For a number of years, however, it has beenevident that a closer control of the Thermit reaction is highlydesirable it not essential in order to adapt this process to the manyuses for which it has been recommended.

While Goldschmidt substantially reduced the explosive character of theThermit reaction by ignitin'g the mass at one point instead of byheating the crucibleuntil reaction commenced, the ensuing reactionnever-the-less has always been violent as well as erratic. usual Thermitreaction, produced by the ignition at one point of a cold mass ofThermit in a crucible, include a variation in the time as well'as theeificiency of the reaction. The slower reactions have the lowerefllciencies. And it has been found that the loss of emciency whichaccompanies the slow reactions results to at least some extent in a lossof Thermit, which goes into solution in the steel and the slag' withoutre acting. Any iron oxide or aluminum which may dissolve in the steel inthis manner is, of course, detrimental. But a more serious result oftheerratic nature of the Thermit reaction, as usually conducted, hasbeen the fact that the temperature of the Thermit metal obtained couldnot be accurately predetermined.

The superheat obtained in the Thermit metal and molten slag is-one ofthemost valuable of the products recovered in the Thermit reaction. Thissuperheat is, of course, what makes Thermit welding possible. But in thewelding process it is necessary to preheat the parts to be welded and,unless the temperature of the Thermit metalcan be accuratelypredetermined, it is impossible to determine to what temperature theparts should be preheated. Underheating causes insuflicient welding andoverheating results in the burning of the metal.

The shorter the time of the Thermit reaction,

the hotter the Thermit metal which is obtained.

I have discovered a methodof igniting Thermit mixtures in the cruciblewhich results in a ,reaction of substantially reduced violence in spiteof the fact that the reaction time is reduced.

While studying the cause for the extreme violence The vagarities of theproduced at the end of the reaction, when a crucible of Thermit powderis ignited in accordance with the practice first suggested by HansGoldschmidt, I found that the course of the reaction in the crucible isabout as follows: The ignition powder ignites the Thermit at one pointforming a mass ofThermit metal. This molten mass of metal, being heavierthan the mixture, falls downwardly through the powder and ignites theThermit with which it comes in contact on its way to the bottom of thecrucible and, of course, increasing in size on its downward path. Theignition is also propagated in a horizontal plane by what might becalled a progressive or chain'reaction, the reaction of particles in thecharge causing the liberation of sufficient-heat to cause the reactionof the adjacent particles. But this occurs at a rate which isconsiderably slower than the rate of fall of the mass of molten metal.The

result is that a substantial pool of molten metal 'schmidt. Theunreacted mass of Thermit remaining in the crucible therefore reactswith extreme violence and may even throw someof the molten products outof the crucible.

I have found that any substantial heating of the Thermit mass from belowmust be avoided, if this type of reaction is to be eliminated. :In otherwords, the zone of ignition should be propagated horizontally ordownwardly but not upwardly, which means that the Thermit mixture in thetop of the crucible must'be substantially reacted before any substantialpool of metal collects in the bottom of the crucible. And I have foundthat this result-can be accomplished in a convenient manner with butlittle change in the usual technique of igniting the crucible ofThermit.

I have found that, if the entire top surface of the Thermit mixture inthe crucible is ignited substantially at one time, this produces ahorizontal zone of ignition which travels downwardly through thecrucible causing the substantially complete reaction of the Thermitduring its progress to the bottom of the crucible. The progressive orchain type of reaction is preserved throughout. The Thermit is uniformlyand progressively ignited from the top downwardly and therefore heatingfrom below is avoided. There is no unreacted Thermit mixture to beheated en masse to reaction temperatures and thereby caused to reactwith the usual violence. A more uniform, albeit a more rapid, reactionresults. The time of reaction is reduced from 25 to 40 per cent,depending upon the size and shape of the crucible. And; surprisingly,this time of reaction is accurately reproducible in spite of itsshortness. There is no loss of Thermit or slag from the crucible andthe, composition of the Thermit metal can therefore be predeterminedwith a degree of accuracy hitherto unobtainable. The Thermit metalobtained is at a higher temperature and the. temperature obtained insucceeding reactions is uniform and therefore can beaccuratelypredetermined.

The unexpected results described above can be obtained, as mentioned, byigniting substantially the entire top surface of the Thermit mass at onetime. This .is, of course, most easily accomplished by the use of anignition powder which is spread over the top surface of the Thermit inthe crucible. But in order to obtain the desired results it is notnecessary that the ignition powder cover the entire surface of theThermit mass but only that it be distributed across this surface in suchmanner that the top surface is completely ignited before the formationof any substantial pool of Thermit metal in the bottom of the crucible.I have found that this can be accomplished, for example, by placing theignition powder on top of the-Thermit .in the shape of a cross, the armsof which extend substantially to the wall of the crucible. It is alsopossible to secure the desired result by the use of a ring of ignitionpowder spaced approximately midway between the center and the wall ofthe crucible.

In conducting the present invention any of the usual ignition powderscan be employed. In the Goldschmidt patent it is suggested that suitableignition powders may be made by mixing powdered aluminum with easilyreducible compounds, such as lead oxide and barium peroxide. It isusually advantageous to use the oxidizing agent and aluminum in amountswhich are substantially chemically equivalent and improved results areobtained if a little of the Thermit powder to be ignited isincorporated. The ignition powders thus produced have a rate ofcombustion which is substantially greater than that ofthe Thermitproper. This ignition powder can be spread loosely on top of the Thermitin the crucible or it may be compressed into the form of briquets of anydesired shape. The ignition powder can be ignited by means of ;amagnesium band, as described in the Goldschmidt patent, or by means of amatch, or by touching it with a hot iron rod.

My invention can be described more specifically in connection with theaccompanying drawing which shows, more or less diagrammatically, avertical cross section through a Thermit crucible which is charged witha Thermit and ignition powder, ready to be ignited in accordance withthe process of thisinvention. The figure is provided with descriptivelegends which are believed to be self explanatory.

The crucible shown is constructed with the usual magnesia lining and isprovided with a tapping pin at the bottom. An asbestos washer is placedabove the tapping pin and a metal disc, adapted to be fused by thereaction products, is placed above the washer. The Thermit charge flllsthe bulk of the crucible and on top of this is shown the ignition powderwhich, in the embodiment illustrated, is in the form of a layer-coveringthe top of the charge. The crucible is usually provided with a coverwhich is not shown.

While I have described what I consider to be the best embodiments of myprocess, it is obvious, of course, that the various specific. procedureswhich have been described can be altered in many particulars withoutdeparting from the purview of this invention. My invention can beapplied in the case of all of the usual types of Thermits. It isparticularly applicable to the ordinary iron Thermit which is widelyused in the Thermit weldingart.

While I have mentioned that the ignition powder can be distributedentirely across the top of the Thermit or in the form of a cross or aring, it is possible, of course, to devise addi- A series of concentricrings can be used, for example, or the ignition powder may be merelyplaced in spaced points across the top of the Thermit. As statedpreviously it is only necessary that the top of the Thermit mass beignited substantially completely and simultaneously in such manner as toform a horizontal zone of ignition which then .travels downwardly to thebottom of the crucible while reacting the Thermit in its path, heatingof the Thermit from beneath being thus substantially avoided. In otherwords, the reaction must be conducted in such a way that the mass ofThermit is substantially reacted before the formation of any substantialpool of molten Thermit metal at the bottom of the crucible.

propagating, horizontal zone of ignition is formed having an areasubstantially coinciding with the cross'section of the crucible,resulting in a Thermitv reaction of substantially. uniform intensity anda Thermit metal having a maximum, predetermined temperature.

2. The process of igniting Thermits which 1 comprises placing on the topof a mass of Thermit an ignition powder having a rate of combustionwhich is-sub'stantially greater than that of said Thermit, said ignitionpowder covering at least a large portion of the top surface of saidmass, being in contact with saidmass and being distributed in suchfashion as to produce, when. ignited, a self-propagating, horizontalzone of ignition substantially coinciding with the cross section of saidThermit mass, then igniting said ignition powder,- thereby producing aThermit reaction of substantially uniform intensity and a Thermitmetalhaving a maximum, predetermined temperature.

3. The process of igniting Thermits which comprises placing a mass ofThermit in a crucible, placing on the top surface of the mass anignition powder having a rate of combustion which is substantiallygreater than that of said Thermit, said ignition powder being disposedin the form of a cross, the arms of which extend close to the wall ofthe crucible, and igniting the ignition powder thereby producing aself-propagating, horizontal zone of ignition substantially coincidingwith the cross section of the crucible,

resulting in the production of a Thermit reaction of substantiallyuniform intensity and a Thermit metal having a maximum, predeterminedtemperature.

4. The process of igniting Thermits used in Thermit welding processeswhich comprises placing a mass of an iron'Thermit in a crucible providedwith a tap in such manner as to produce a substantially horizontal topsurface, distributing an ignition powder across a substantial portion ofthe top surface of said mass in contact therewith and in suchconfiguration that, upon ignition, the top surface of said Thermit masswill become completely ignited before the formation of any substantialpool of Thermit metal in the bottom of the crucible, and igniting theignition powder thereby producing a Thermit reaction of substantialuniform intensity and a Thermit metal having a maximum predeterminedtemperature prior to the tapping of the crucible, said ignition powderbeing one havinga rate of combustion substantially greater than that ofsaid Thermit.

FREDERICK w. COHEN;

